A common apparatus for measuring pollutants being emitted from a smokestack is a transmissometer having a transceiver on one side of the smokestack and a reflector on the opposite side of the smokestack for reflecting a light beam projected by the transceiver across the stack. The change in the amount of reflected light is indicative of the opacity created by pollutants in the stack stream. Conventional transmissometers of this type over fill the reflector, i.e., the light beam is much larger in diameter than the diameter of the reflector. As long as the light beam is uniform and overlays the entire reflector, no errors in measurement are incurred. However, when misalignment occurs there is movement of the beam around the reflector so that the resultant returned light similarly moves around on the light detector in the transmissometer. Slight non-uniformities in the sensitivity of the detector surface result in measurement errors like non-uniformities in the measurement beam itself. An additional complication arises from the fact that in the overfilled system, the objective lens typically has to be focused to insure that the plane of maximum light uniformity is in the plane of the reflector. The problem with this conventional technique is that it is very difficult to make the light beam uniform and large enough to substantially over fill the reflector. Furthermore, when the reflector is overfilled, the system becomes more inefficient since only a portion of the outgoing light strikes the reflector and is returned to the transceiver.
United States Environmental Protection Agency (EPA) has established performance specifications for opacity monitors which are used to monitor smoke density and smokestacks at regulated facilities. These specifications are described in 40 C.F.R. 60, Appendix B, Perf. Spec. 1. One of the requirements for opacity monitors is that the alignment detection scheme employed be capable of sensing or indicating when the transmissometer transceiver and reflector become misaligned to such a degree that a 2% error in opacity occurs. On typical smokestack installations or duct installations the support structure for the transceiver and reflector are not completely mechanically stable. As a result, the alignment varies with wind, rain, temperature, shock and vibration which typically is caused by imbalances and associated fans and/or motors, aging and corresponding shifts in foundations, and changes in structural stress caused by changing loads on the associated boiler. Thus, it is not uncommon for misalignment of the transceiver and reflector to occur, and if it is not detected and corrected, it causes a degradation in measurement accuracy. The corresponding measurement error results from the light beam, which has varying degrees of spatial non-uniformity, moving around on the reflector surface and in some cases even moving partially off of the reflector surface.
A current practice used for detecting misalignment in an overfilled system is to provide a bulls eye on a frosted glass window upon which the return beam is focused. In some designs, this alignment target is solenoid activated for positioning the bulls eye in the light path when needed for alignment purposes. During normal measurement, the bulls eye is moved out of the path of the light beam. In other designs the target is activated by light from a beam splitter which always receives a portion of the reflected light intensity. With these arrangements, an operator has to be physically present at the transmissometer installation site, which may be well up on the side of the stack. In addition, the operator must peer into an alignment port where he can observe the returned light on a bulls eye screen. Then he must determine if the alignment condition is satisfactory or so far off center to justify realigning the system. This is a subjective evaluation since the observance of the light on the bulls eye is not highly definitive. Further, if there is a lot of thick smoke in the stack and the stack is physically large, it is difficult to see the returned light spot. As a result, the alignment of such apparatus is only rarely checked and even then the ability to decern proper alignment is marginal. Consequently, many such systems continue to operate long after the alignment begins to compromise the measurement accuracy of the system and the operator is unaware of the degradation which occurs.